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...to save the human race (from Evil Energy).
From Comic BonBon Special #118, the Rockman 10 Years History Book, page 66:
Justice's Powerful Fighter from Beyond the Cosmos
A robot who first appeared in the arcade game "Rockman 2: The Power Fighters." in "Rockman 8" some light was shed on his past, revealing that he had been chasing the Evil Energy embodiment through space when he arrived on the Earth, and that he saved Rockman's life from the injuries received during his battle with Dr. Wily. The extent of his abilities are shrouded in mystery, but compared to the likes of Rockman and Forte, his power is thought to far exceed that of the others. It should be interesting to note what kind of appearances he'll make in the series from here on out.
Duo learned of how Rockman fought all along to protect peace on Earth, and used his miraculous power to save Rockman's life.
At first, Duo thought Rockman was trying to interfere with his mission, and so they met in battle. That power was astonishing!